This invention relates to high speed, high power silicon controlled rectifiers, and more specifically relates to a novel silicon controlled rectifier and process for manufacture thereof which places the flat upper emitter contact surface at a higher level than its interdigitated gate contact surface to enable a flat expansion plate contact to be laid into high pressure contact with the emitter contact without making connection to the gate contact of the device.
High speed, high power controlled rectifiers in which contact is made to the emitter contact layer by a molybdenum expansion plate or the like are well known. Such devices are shown, for example, in U.S. Pat. No. 3,609,476.
In order to make the silicon controlled rectifier or thyristor operate at as high a speed as possible, it is preferable to interdigitate the gate and emitter contacts so that the device will quickly turn on over its full area as rapidly as possible and thus be capable of operation with a high rate of rise of anode to cathode current. When the device is to have high current capacity and is made of a relatively large area wafer, it is also desired to make pressure contact to the device through an expansion plate such as a disk of molybdenum or tungsten or other material having thermal coefficient expansion characteristics similar to those of silicon. The expansion plate is pressed into high pressure contact with the emitter contact. A gate lead is then connected to the gate area which is interdigitated with the emitter.
A problem exists in making pressure contact to the emitter contact of a high current silicon controlled rectifier without also making contact to the interdigitated gate contact, thus short-circuiting the gate electrode to the emitter.
This connection has been made in the past by the use of a conductive shim having the configuration of the interdigitated emitter contact, with the shim laid atop the emitter and receiving the flat surface of an expansion contact. Because of the presence of the shim, the expansion plate does not contact the gate electrode which is interdigitated with and generally coplanar with the emitter contact. Such shims require exact orientation and one must maintain exact orientation between the shim and the emitter during assembly of the device. Moreover, it is an added part which must be handled during device assembly.
In other attempts to solve the problem, silk screening of the emitter contact material has been used to build up the emitter contact to a level higher than that of the gate contact. Arrangements such as this are shown in U.S. Pat. No. 4,155,155. A built-up emitter contact configuration which has a higher level than the gate contact which is interdigitated therewith is also shown in U.S. Pat. Nos. 3,609,476 and 4,096,623.
The processes which have been used in the silk screening approach to build up the level of the emitter contact are cumbersome and expensive and difficult to carry out in mass production.
Still another method used in the past involves the etching away of raised levels of the gate contact after the gate and emitter contacts are formed. This requires complex masking and etching processes.